Mr Biden said prisoner negotiations “come with tough calls” but added that “it says a lot about the United States that we work relentlessly to free Americans”.

“The deal that made this possible was a feat of diplomacy and friendship. Multiple countries helped get this done,” he added. “They joined a difficult, complex negotiation at my request. And I personally thank them all again.”

US officials said Mr Biden worked personally on the agreement, and was finalising arrangements with his counterparts in Slovenia one hour before his announcement that he would drop out of the presidential election last month.

He also convinced Olaf Scholz, the German chancellor, to release Krasikov from prison to complete the deal in talks held during Mr Scholz’s visit to the White House in February.

On Thursday, Mr Scholz said the “difficult” decision to release the convicted Russian hitman had “saved lives”.

“Nobody took this decision to deport a murderer sentenced to life imprisonment after only a few years… lightly,” he said at Cologne airport, in western Germany, where some of the released prisoners were due to arrive.

Jake Sullivan, the US national security adviser, said the “historic” prisoner swap had required months of negotiation with Russia and coordination between Western allies to pull off.

He said the deal was “vintage Joe Biden, rallying American allies to save American citizens”, adding: “If you had not had Joe Biden sitting in the Oval Office, I don’t think this would have happened.”